I mean the alleged thefts of supplies happened in 1919 and the Bolsheviks had already classified the Black Army as "Deserters and Bandits" way back in 1917.

The Bolshevik policy towards "Bandits and Deserters" at the time was that they should be shot on sight. They declassified them for a year while they were making common cause against the Whites but then when they decided to turn on the Mahknovists again in 1919 not only did they reclassify them, they upgraded their policy on Deserters and Bandits to, in Lenin's own words

After the expiration of the seven-day deadline for deserters to turn themselves in, punishment must be increased for these incorrigible traitors to the cause of the people. Families and anyone found to be assisting them in any way whatsoever are to be considered as hostages and treated accordingly.

In case you were wondering what "treated accordingly" meant here's a Cheka report

Yaroslavl Province, 23 June 1919. The uprising of deserters in the Petropavlovskaya volost has been put down. The families of the deserters have been taken as hostages. When we started to shoot one person from each family, the Greens began to come out of the woods and surrender. Thirty-four deserters were shot as an example.

The Russian Bolsheviks and the subsequent USSR were a powerful force for good in the world but they should not have done what they did to the Mahknovists.